Hydroalcoholic extraction and carbon-coated zero-valent iron nanoparticles: Comparative valorisation routes for olive mill wastewater

Abstract

This study evaluated two valorisation routes for olive mill wastewater (OMW): hydroalcoholic extraction of a crude phenolic-rich fraction and hydrothermal carbonisation (HTC) to produce carbon-coated zero-valent iron nanoparticles (FeC 0 NPs). Their cradle-to-gate profiles were compared by life cycle assessment (LCA), using evaporation ponding as a semiquantitative benchmark. The LCA showed lower impacts for extraction across all 12 midpoint categories, including climate change (0.552 versus 1.165 kg CO 2 -eq kg −1 OMW). Extraction generated an aqueous effluent stream, whereas HTC required iron salt addition, hydrothermal treatment and graphitisation at 700 °C, generating aqueous and solid residual streams. Ponding involved low electricity use but released substantial chemical oxygen demand and phenolic loads to water; methodological differences precluded direct ranking. Overall, extraction showed lower foreground environmental burdens, while HTC provided a more integrated conversion into a functional solid product at the expense of higher energy and reagent demands. Regarding the upcycling routes and their resulting products, response surface methodology identified 70 °C and 100 min as the constrained optimum within the predefined operating range compatible with industrial operation. The lyophilised OMW extract showed a total phenolic content of 40.7 mg gallic acid equivalents g −1 of lyophilised extract and antiproliferative activity against HeLa cells (IC₅₀ = 3.39% w·v −1 , g lyophilised OMW extract/distilled water). FeC 0 NPs produced species-dependent microbial responses, with the clearest effect being a 3.7-fold increase in the maximum growth rate of Pseudomonas fluorescens .